1. Field of the Invention
The present invention relates to oxidation catalysts, and more particularly to oxidation catalysts which are useful for producing maleic anhydride from C.sub.4 to C.sub.10 hydrocarbons.
2. The Prior Art
Essentially all of the catalysts and methods disclosed in the prior art for producing maleic anhydride from C.sub.4 and higher hydrocarbons employ oxidation catalysts containing vanadium in a valence state of less then +5. One method of forming such catalysts is to impregnate a catalyst base with a vanadium compound in which the vanadium has a valence of less then +5. Another more desirable method involves impregnating the catalyst with a vanadium compound which has vanadium in the +5 valence state and then reducing the vanadium from the +5 valence state to a valence less than +5.
Several references disclose oxidation catalysts containing vanadium-phosphorus mixed oxide catalysts and methods of preparing the same. For example, U.S. Pat. No. 3,243,385 discloses a process for preparing a catalyst containing vanadium, tin and optionally phosphorus deposited on a carrier material. Silicic acid is described as one desirable carrier in the patent.
U.S. Pat. No. 3,867,411 relates to an oxidation process for producing maleic anhydride from C.sub.4 to C.sub.10 saturated hydrocarbons utilizing as the oxidation catalyst a composite of phosphorus, vanadium, iron, oxygen and chromium.
U.S. Pat. No. 4,061,724 describes a crystalline silica composition prepared by calcining a crystalline hydrated alkylonium silicate prepared hydrothermally from a reaction mixture containing water, amorphous silica and a quarternary ammonium compound.
U.S. Pat. No. 4,064,070 discloses a catalyst comprising vanadium, phosphorus and silicon oxides prepared by coprecipitating vanadium oxide and silica or a silica precursor. The catalyst is described as suitable for use in oxidizing a hydrocarbon feed to produce maleic anhydride.
Finally, U.S. Pat. No. 4,151,116 relates to a catalyst for oxidizing butane to produce maleic anhydride, said catalyst comprising a substrate containing phosphorus and vanadium, and optionally containing an activator and a post deposited promoter. Substrates described as suitable for use include silica gel, silica alumina, silica etc. Suitable activators include compounds of titanium, such as the halides, phosphates, sulfates and the like. The post deposited promoter is preferably a zinc salt.
As can readily be determined from the above, there is an ongoing effort to develop oxidation catalysts for preparing maleic anhydride from alkanes and olefins.
Accordingly, it is an object of the present invention to provide a large surface area oxidation catalyst for producing maleic anhydride and a method for preparing the same.
A further object of the present invention is to provide a method for obtaining improved yields and selectivity of maleic anhydride and, in addition, improvements in catalyst stability.
Another object of the present invention is to provide an improved oxidation catalyst for oxidizing saturated and unsaturated hydrocarbons to maleic anhydride.
These and other objects are accomplished according to the present invention by oxidizing either a saturated or unsaturated hydrocarbon having from four to ten carbon atoms in the presence of an oxidation catalyst comprising vanadium, phosphorus, tin and a crystalline silica.